The growing popularity of digital multimedia applications (e.g., short message service, internet, WebTV, etc.) has created an ever increasing demand for digital broadband communication systems. Consequently, more and more user data is transmitted over limited frequency bands to increase system throughput and capacity using various transmission techniques. However, these broadband transmission techniques are very sensitive to transmission impairments such as noise, adjacent channel interference, inter-symbol interference, multipath effects, and other impairments.
Growing more popular recently (especially in the United States), orthogonal frequency division multiplexing (OFDM) has been used to help solve these negative effects from multiple user digital broadband transmission. OFDM, chosen as the transmission method for European radio (DAB—Digital Audio Broadcasting) and TV (DVB-T—Digital Video Broadcasting) standards, is a multicarrier transmission technique that divides the available spectrum into many carriers, each one being modulated by a low rate data stream. Similar to Frequency Division Multiple Access (FDMA), OFDM achieves multiple user access by subdividing the available bandwidth into multiple narrowband channels that are allocated to users.
However, OFDM uses the spectrum more efficiently by spacing the channels much closer together (actually overlapping). This close spacing of user channels is achieved by making all the carriers orthogonal to one another which prevents interference between the closely spaced carriers. This orthogonal relationship is created (using an IFFT—Inverse Fast Fourier Transform) by each carrier having an integer number of cycles over a symbol period. As shown in FIG. 1, due to this periodicity, the spectrum of each carrier (a, b, c, d) has a null at the center frequency of each of the other carriers in the system resulting in no interference between carriers, and allowing the carriers to be as closely spaced as theoretically possible. At the receiving end, each carrier (or subcarrier) may be evaluated at a particular frequency (or time period) and all other carrier signals should be zero (eliminating adjacent carrier interference). Additionally, as shown in FIG. 2, to help combat multipath effects a guard period 205 is added between transmitted symbols (information transmitted over the carriers) which is most commonly a combination of a cyclic extension of the symbol and a zero amplitude signal (no-signal period).
A key factor in preserving the orthogonal relationship between carriers is synchronization (operating on same modulation frequency and time-scale) between transmitter and receiver. Commonly, synchronization is maintained (recovered) at the receiver end by detecting this no-signal (null) period of the transmitted OFDM signal by comparing the power (energy) of a received OFDM signal with multiple pre-determined threshold levels. The pre-processed signal 805 and processed (filtered) signal 810, including null period detection 815, are shown in FIG. 3.
Commonly, in practicing this technique, detected maximum and minimum signal levels are set as thresholds and then negatively and positively adjusted, respectively, to find the exact point (ramp-up period for a timing reference) for the start of the transmitted data frame (portion) following the guard period 815. For example, with reference to FIG. 3, a first maximum level (threshold) of 10 may be found at time 380, and a first minimum level (threshold) of 0.5 may be found at time 255 for filtered signal 810. Thereafter, the maximum level threshold may be repeatedly reduced (e.g., initially by 50%) and the minimum level threshold may be repeatedly increased (e.g., initially by 25%) to find this exact starting point for the transmitted data frame to recover time synchronization. U.S. Pat. No. 6,246,735, the disclosure of which is incorporated by reference herein, provides further description of detecting the end of the null period using successive calculation of power (energy) thresholds. However, this threshold-adjustment technique requires complicated calibration of multiple thresholds and calculation of power (energy) levels making it susceptible to noise and very sensitive to the shape of the no-signal period.
Therefore, due to the disadvantages of current synchronization approaches, there is a need to provide a synchronization method that efficiently recovers a timing reference (e.g., no-signal period) from the received OFDM signal in the presence of noisy conditions, and does not depend on the shape of the no-signal period nor require complicated calibration of multiple energy level thresholds.